1. Technical Field
The invention relates to a wireless communication system, a wireless communication apparatus and a wireless communication method using spatial multiplexing, and more particularly, to a wireless communication system, a wireless communication apparatus and a wireless communication method, in which a transmitter and a receiver share channel information to perform closed loop type spatial multiplexing transmission.
In particular, the invention relates to a wireless communication system, a wireless communication apparatus and a wireless communication method, which perform beamforming on the basis of information which is fed back from a receiver when a transmitter transmits a packet, and more particularly, to a wireless communication system, a wireless communication apparatus and a wireless communication method, which perform beamforming by feeding back beamforming information between a beamformer and beamformee which are different from each other in the number of antennas or the number of supported streams.
2. Background Art
As a system for removing wire in an existing wired communication method, a wireless network is attracting attention. A standard of the wireless network may be the IEEE (The institute of Electrical and Electronics Engineers) 802.11 or the IEEE 802.15.
For example, in the IEEE 802.11a/g, as a standard of a wireless LAN, an orthogonal frequency division multiplexing (OFDM) modulation method which is one of a multi-carrier method is employed. In the OFDM modulation method, since transmission data is distributed to a plurality of carriers having orthogonal frequencies and is transmitted, the band of each carrier becomes narrow, frequency use efficiency is very high, and frequency-selective fading interference is strong.
In addition, in the IEEE 802.11a/g standard, a modulation method for accomplishing a communication speed of a maximum of 54 Mbps is supported, but a next-generation wireless LAN standard for realizing a new high bit rate is required.
As one of a technology of realizing a high speed of wireless communication, multi-input multi-output (MIMO) communication is attracting attention. This is a communication method in which both a transmitter side and a receiver side respectively include a plurality of antennas to realize spatially multiplexed streams. The transmitter side performs spatial/temporal encoding and multiplexing of plural pieces of transmission data and distributes and transmits the plural pieces of transmission data to N transmission antennas through channels. The receiver side performs spatial/temporal decoding of reception signals received by M reception antennas through the channels to obtain reception data without crosstalk between the streams (for example, see JP-A-2002-44051 (Patent Document 1)). Ideally, spatial streams corresponding to the smaller number (MIN[N, M]) of the transmission and reception antennas are formed.
According to the MIMO communication method, a transmission capacity can increase according to the number of antennas and a communication speed improvement can be realized, without increasing a frequency band. Since the spatial multiplexing is used, frequency use efficiency is high. The MIMO method uses channel characteristics and is different from a simple transmission/reception adaptive array. For example, in the IEEE 802.11n which is the extension standard of the IEEE 802.11a/g, an OFDM_MIMO method using OFDM in primary modulation is employed. Currently, the IEEE 802.11n is being standardized in a task group n(TGn) and a specification established therein is based on a specification established in Enhanced wireless consortium (EWC) formed on October, 2005.
In the MIMO communication, in order to spatially divide a spatially multiplexed reception signal y into the stream signals x, a channel matrix H is acquired by any method and the spatially multiplexed reception signal needs to be spatially divided into a plurality of original streams using the channel matrix H by a predetermined algorithm.
The channel matrix H is obtained by allowing a transmitter/receiver side to transmit/receive existing training sequence, estimating the channels by a difference between the actually received signal and the existing sequence and arranging propagation channels of a combination of transmission and reception antennas in a matrix form. When the number of transmission antennas is N and the number of reception antennas is M, the channel matrix is M×N (row×column) matrix. Accordingly, the transmitter side transmits N training sequence and the receiver side acquires the channel matrix H using the received training sequence.
A method of spatially dividing a reception signal is largely classified into an open loop type method in which a receiver independently performs spatial division on the basis of the channel matrix H and a closed loop type method in which a transmitter side gives weights to the transmission antennas on the basis of the channel matrix to perform adequate beamforming toward a receiver to form an ideal spatial orthogonal channel.
As an open loop type MIMO transmission method, there is a zero force (for example, see A. Benjebbour, H. Murata and S. Yoshida, “Performance of iterative successive detection algorithm for space-time transmission”, Proc. IEEE VTC Spring, vol. 2, pp. 1287-1291, Rhodes. Greece, May 2001 (Non-Patent Document 2)) or a minimum mean square error (MMSE) (for example, see A. Benjebbour, H. Murata and S. Yoshida, “Performance comparison of ordered successive receivers for space-time transmission”, Proc. IEEE VTC Fall, vol. 4, pp. 2053-2057, Atlantic City, USA, September 2001 (Non-Patent Document 3)). The open loop type MIMO transmission method is a relative simple algorithm for obtaining reception weight matrix W for spatially dividing the reception signal from the channel matrix H, in which a feedback operation for sharing the channel information between the transmitter and the receiver is omitted and the transmitter and the receiver independently perform spatial multiplexing transmission.
As an ideal one of a closed loop type MIMO transmission method, a singular value decomposition (SVD)-MIMO method using SVD of the channel matrix H is known (for example, see http://radio3.ee.uec.ac.jp/MIMO(IEICE_TS). Pdf (Oct. 24, 2003) (Non-Patent Document 1)). In the SVD-MIMO transmission, a numerical matrix having channel information corresponding to antenna pairs as elements, that is, a channel information matrix H, is subjected to the singular value decomposition to obtain UDVH. A transmitter side uses V in a transmission antenna weight matrix and transmits a beamformed packet to a receiver and a receiver side typically gives (UD)−1 as a reception antenna weight matrix. Here, D is a diagonal matrix having square roots of singular values λi corresponding to qualities of the spatial streams in diagonal elements (the subscript i indicates an ith spatial stream). The singular values λi are arranged in the diagonal elements of the diagonal matrix D in ascending order and power ratio distribution or modulation method allocation is performed according to communication quality represented by the level of the singular value with respect to the streams such that a plurality of spatial orthogonal multiplexed propagation channels which are logically independent are realized. The receiver side can extract a plurality of original signal sequence without crosstalk and theoretically accomplish maximum performance.
In the closed loop type MIMO communication system, adequate beamforming is performed when the transmitter transmits the packet, but information on the channel information needs to be fed back from the receiver side for receiving the packet.
For example, in the EWC HT (High Throughput) MAC (Media Access Control) Specification Version V1.24, two kinds of procedures, that is, “implicit feedback” and “explicit feedback”, are defined as the procedure for feeding back the information on the channel matrix between the transmitter and the receiver.
In the “implicit feedback”, the transmitter estimates a backward channel matrix from the receiver to the transmitter using training sequence transmitted from the receiver, and a forward channel matrix from the transmitter to the receiver is computed to perform beamforming on the assumption that bidirectional channel characteristics between the transmitter and the receiver are reciprocal.
In the “explicit feedback”, the receiver estimates a forward channel matrix from the transmitter to the receiver using training sequence transmitted from the transmitter and returns a packet including the channel matrix as data to the transmitter, and transmitter performs the beamforming using the received channel matrix. Alternatively, the receiver computes a transmission weight matrix for allowing the transmitter to perform the beamforming from the estimation channel matrix and returns a packet including the transmission weight matrix as the data to the transmitter. In the explicit feedback, since the weight matrix is computed on the basis of the estimated forward channel matrix, it may not be assumed that the channels are reciprocal.
In view of packet transmission, the transmitter is an initiator and the receiver is a receiver. However, in view of beamforming, the initiator for transmitting the packet is a beamformer and the receiver for receiving the beamformed packet is a beamformee. Communication from the beamformer to the beamformee is referred to as “forward” and communication from the beamformee to the beamformer is referred to as “backward”. For example, when an access point (AP) transmits a data frame to a client terminal (STA) as the beamformer, the access point perform the beamforming on the basis of the channel information transmitted from the client in the explicit feedback.
FIG. 14 shows a state where the beamformee estimates the channel matrix excited by a training signal transmitted from the beamformer. In the same drawing, a STA-A having three antennas is the beamformer and a STA-B having two antennas is the beamformee and feedback is performed on the basis of a CSI format. In the below-described description or equations, a subscript AB indicates forward transmission from the STA-A to the STA-B. A numerical subscript corresponds to the antenna number of the corresponding terminal.
The training sequence transmitted from the antennas of the STA-A are (tAB1, tAB2, tAB3) and the signals received by the antennas of the STA-A through a channel HAB are (rAB1, rAB2), the following equation is obtained.
                              (                                                                      r                                      AB                    ⁢                                                                                  ⁢                    1                                                                                                                        r                                      AB                    ⁢                                                                                  ⁢                    2                                                                                )                =                              H            AB                    ⁡                      (                                                                                t                                          AB                      ⁢                                                                                          ⁢                      1                                                                                                                                        t                                          AB                      ⁢                                                                                          ⁢                      2                                                                                                                                        t                                          AB                      ⁢                                                                                          ⁢                      3                                                                                            )                                              (        1        )            
where, the channel matrix HAB is a 2×3 matrix and expressed by the following equation. But, hij is a channel characteristic value of Jth antenna of the STA-A to ith antenna of the STA-B.
                              H          AB                =                  (                                                                      h                  11                                                                              h                  12                                                                              h                  13                                                                                                      h                  21                                                                              h                  22                                                                              h                  23                                                              )                                    (        2        )            
When the channel matrix HAB is subjected to singular value decomposition, the following equation is obtained. Here, UAB is a matrix having an inherent normalized vector of HABHABH, VAB is an inherent normalized vector of HABHHAB and DAB is a diagonal matrix having a square root of an inherent vector of HABHABH or HABHHAB as the diagonal elements. In addition, UAB and VAB are unitary matrices and complex conjugate transposed matrices thereof become inverse matrices.HAB=UABDABVABH  (3)
The transmission weight matrix necessary for forming the frame transmitted from the STA-A to the STA-B is the matrix VAB obtained by performing the singular value decomposition with respect to the forward channel matrix HAB. When the beamformee receives a sounding packet, the beamformee divides the sounding packet into spatial stream trainings to construct the estimation channel matrix HAB. The CSI composed of MIMO channel coefficients h11, h12, . . . which are elements of the channel matrix is collected and fed back to the STA-A.
If a transmission vector composed of transmission signals of the antennas of the STA-A is x and a reception signal of the STA-B is y, the reception signal becomes y=HABx in a case where the beamforming is not performed (un-steered), but the reception signal y becomes the following equation in a case where the beamforming are performed by the transmission weight matrix VAB (steered).
                    y        =                                            H              AB                        ⁢                          V              AB                        ⁢            x                    =                                                                      (                                                            U                      AB                                        ⁢                                          D                      AB                                        ⁢                                          V                      AB                      H                                                        )                                ·                                  V                  AB                                            ⁢              x                        =                                          U                AB                            ⁢                              D                AB                            ⁢              x                                                          (        4        )            
Accordingly, the STA-B can perform spatial division to the original stream by multiplying a reception vector including the reception signals of the antennas by DAB−1UABH as a reception weight.
FIG. 15 shows a frame exchange procedure for transmitting beamforming from the access point to the client terminal by the explicit feedback.
This procedure is initiated by the access point which sends the sounding packet including a CSI feedback request.
The sounding packet includes the training sequence excited by the channel matrix. Accordingly, when the sounding packet is received, the client terminal divides the spatial stream training to estimate the channel matrix H and collects the CSI. The CSI data is included in the packet as a CSI feedback (CFB) and returned to the access point.
The access point computes the transmission weight matrix for beamforming from the received CFB and multiplies the transmission signal by it to transmit the beamformed packet to the client terminal. Even in a place where the communication was hard to be accomplished in the past, communication is accomplished at a high transmission rate by the beamforming.
As described above, in the explicit feedback, the beamformer can receive the explicit feedback of the estimation channel matrix from the beamformee. The format of the feedback format of the estimation channel matrix is largely classified into a case where an MIMO channel coefficient is sent and a case where a transmission weight matrix V for beamforming computed by the beamformee.
The former format is called channel state information (CSI). The beamformer needs to compute the transmission weight matrix V for beamforming by constructing the channel matrix H from the received CSI and performing the singular value decomposition.
The latter is classified into a case where the transmission weight matrix V for beamforming is sent in an uncompressed format and a case where the transmission weight matrix V for beamforming is sent in a compressed format. According to the explicit feedback, a processing burden for estimating the channel matrix in the beamformer side and a processing burden for calculating the transmission weight matrix from the channel matrix are reduced.
FIG. 16 shows a scheme of a HT control field of an MAC frame defined in the EWC specification. The HTC field has 32 bits, but, among them, 22nd to 23rd CSI/steering fields can specify a feedback type received from the beamformee in the explicit feedback (see FIG. 17).
As described above, the processing burden of the beamformer which performs beamforming with respect to a transmission frame is reduced by the explicit feedback. However, when the beamformer and the beamformee are different from each other in the number of antennas or the number of supported streams, several problems are caused at the time of beamforming.
In a spatial multiplexing type communication apparatus, the dimension number which is allowed by the processing capability including the estimation of the channel matrix H, the computation of the transmission weight matrix for beamforming, and the multiplication of the transmission vector and the transmission weight matrix V for beamforming is generally designed according to the number of antennas included therein. Accordingly, the transmission weight matrix for beamforming cannot be constructed by spatially dividing a training signal transmitted from the beamformer having the number of antennas which is larger than an allowable dimension, the transmission weight matrix for beamforming cannot be computed from the channel matrix which is fed back from the beamformee, or the transmission weight matrix for beamforming which is fed back from the beamformee cannot be multiplied with the transmission vector.
First, consider a case where the explicit feedback is performed with a CSI format.
In a case where the number N of antennas of the STA-A is smaller than or equal to the number M of antennas of the STA-B, no problem is specially caused in the beamformee side. FIG. 18 shows a state where the explicit feedback is performed with a CSI format when N=2 and M=3. The STA-B includes a processing capability of M streams, and can estimate an M×N channel matrix excited by a training signal including N streams and feed back the collected CSI information to the STA-A. The STA-A side can suppress the fed-back M×N channel matrix to a range of N rows and compute the transmission weight matrix for beamforming by the singular value decomposition from the N×N channel matrix.
However, in a case of N>M, problems are caused. This is because, when the STA-B can process only M streams, the STA-B obtains only M×M estimation channel matrix using M packets although the STA-A side transmits the sounding packet for exciting N-dimensional spatial channel matrix. FIG. 19 shows a state where the explicit feedback is performed with the CSI format when N=3 and M=2.
In the EWC specification, when the explicit feedback is applied, a scheme of informing information on channel estimation maximum dimension is defined as one of the capabilities of the beamformee side. It is defined that the HT terminal corresponding to high-speed transmission declares that it itself is the HT terminal by including a HT capability field in a predetermined management frame.
FIG. 20 shows a format of a HT capability element. In a TxBF (transmit beamforming) capability field, any HT function of the beamforming is specified. FIG. 21 shows the configuration of the Tx beamforming capability field. The Tx beamforming capability field has 32 bits, but, among them, 19th to 20th bits are allocated to the CSI number of beamformer antennae, 21st to 22nd bits are allocated to the uncompressed steering matrix of beamformer antennae, and 23rd to 24th bits are allocated to the compressed steering matrix of beamformer antennae. In these fields, the spatial dimension number of the sounding packet which can be received from the beamformer when the beamformee performs the explicit feedback with each format is described.
However, in the EWC specification, since it is not defined which sounding packet is transmitted by the beamformer, the STA-A may transmit the sounding packet for exciting more than M channels even when the STA-B informs of its own maximum dimension number by the above-described scheme and thus the STA-B is forced to estimate M×N channel matrix.
As a method of solving such problems without deteriorating the beamforming characteristics, it may be considered that a channel estimation maximum dimension Nmax corresponding to a rated maximum number of antennas is given to the STA-B as the beamformee (for example, if it is based on the IEEE specification, Nmax=4).
For example, when the number of antennas of the STA-B is M=2 and the rated maximum number of antennas is Nmax=4, the STA-B can compute only a 2×2 matrix in consideration of the communication with the terminal having the same number of antenna, but needs to compute a 2×4 matrix. In this case, since calculation or processing circuit needs to be doubled, miniaturization or low cost of the apparatus is hard to be realized.
The same is also applied to the explicit feedback for feeding back the transmission weight matrix V for beamforming, instead of the CSI format.
In a case where the number N of antennas of the STA-A is smaller than or equal to the number M of antennas of the STA-B, no problem is specially caused in the beamformee side. FIG. 22 shows a state where the transmission weight matrix V for beamforming is fed back by the explicit feedback when N=2 and M=3. The STA-B includes a processing capability of M streams, and can estimate an M×N channel matrix excited by a training signal including N streams, compute an N×M transmission weight matrix V for beamforming by the singular value decomposition from the estimation channel matrix, and feed backs the transmission weight matrix information to the STA-A. The STA-A side can perform beamforming using the fed-back transmission weight matrix for beamforming.
However, in a case of N>M, problems are caused. This is because, when the STA-B can process only M streams, the STA-B obtains only an M×M estimation channel matrix using M packets although the STA-A side transmits the sounding packet for exciting N-dimensional spatial channel matrix. FIG. 23 shows a state where the transmission weight matrix V is fed back by the explicit feedback when N=3 and M=2.
In the EWC specification, when the explicit feedback is applied, a scheme of informing information on channel estimation maximum dimension is defined as one of the capabilities of the beamformee side (described above). However, the STA-A may transmit the sounding packet for exciting more than M channels even when the STA-B informs of its own maximum dimension number by the above-described scheme and thus the STA-B is forced to estimate M×N channel matrix.
As a method of solving such a problem without deteriorating the beamforming characteristics, it may be considered that a channel estimation maximum dimension Nmax corresponding to a rated maximum number of antennas is given to the STA-B as the beamformee (for example, if it is based on the IEEE specification, Nmax=4) and a processing capability which can compute the transmission weight matrix for beamforming is given to the obtained Nmax×N estimation channel matrix.
For example, when the number of antennas of the STA-B is M=2 and the rated maximum number of antennas is Nmax=4, the STA-B can compute only a 2×2 matrix in consideration of the communication with the terminal having the same number of antenna, but must compute a 2×4 matrix. In this case, since calculation or processing circuit needs to be doubled, miniaturization, low cost and low power consumption of the apparatus are hard to be realized.